Field of the Invention
This invention relates to a boring bar for suppressing vibrations caused in machining processes and, more particularly, to a tunable boring bar, in which at least a portion of the bar body is clad with a material having a high stiffness.
Description of Related Art
During a metalworking operation, there is relative motion between a workpiece and a cutting tool being urged against the workpiece. Specifically, the surface finish left on the workpiece by a previous pass of the cutting tool creates variation in chip thickness that, in turn, creates fluctuation of the cutting force magnitude. The relative motion between the workpiece and the tool is magnified by this fluctuation of the cutting force and may lead to an unstable condition known as chatter. Chatter is an example of self-excited vibration. As a result of this vibration, a poor quality surface finish and an out-of-tolerance finished workpiece may be produced.
Chatter may be especially problematic when the cutting tool is coupled to an elongated boring bar. A boring bar is essentially a cantilevered member, which is anchored at one end and attached to the cutting tool at the other end. Boring bars are conventionally formed from a metal alloy, such as, carbon steel. To reduce vibrations of the boring bars, cutting parameters such speed and depth of cut may be reduced, decreasing the metal removal rate. However, this approach interferes with production output leading to low productivity.
Numerous attempts to eliminate boring bar vibration are known. One method for reducing vibration is using a boring bar fabricated from a stiffer material, such as solid carbide (e.g., tungsten carbide). However, solid carbide boring bars are more expensive than conventional steel bars. Furthermore, with solid carbide boring bars, although chatter and vibration are reduced by the inherently high stiffness of the solid carbide bar, vibration may still build to an unacceptable level. Additionally, solid carbide is fairly brittle and a minor impact upon the boring bar during use or setup may inadvertently damage the bar. A carbide boring bar extending between a steel adapter and steel tip portion is disclosed in U.S. Pat. No. 6,935,816 to Lee, et al.
Another attempt to reduce vibration in boring bars is by attaching a dynamic vibration absorber mechanisms to or within the boring bar. The dynamic vibration absorber may be used for tuning the boring bar. A dynamic vibration absorber for use in a tunable boring bar, comprised of a cylindrical mass of a high density material supported on resilient bushings, is disclosed in U.S. Pat. No. 3,774,730. When optimally tuned, the mass oscillates in response to vibration produced in the boring bar to cancel out vibration. The absorber may be tuned to accommodate the boring bar for the changes in, the length of the boring bar and the weight of cutting tool connected at the end of the bar. Such an adjustment is made by longitudinally urging pressure plates at opposing ends of the cylindrical mass thereby compressing the rubber bushings against the mass, which alters the stiffness of the rubber supports to change the frequency of the cylindrical mass. Generally, the process of tuning the boring bar is easier for boring bars having higher natural frequencies, where smaller tuning masses can be applied. Therefore, shorter and stiffer bars are typically easier to tune than longer more flexible bars. Tunable boring bars are typically formed from materials that can be machined, such as carbon steel, so that the bar can be fitted to accommodate the vibration absorption mechanism. Therefore, tunable boring bars generally are not made from stiffer materials, such as carbide, which cannot be machined through conventional means. In addition to tunable boring bars, some boring bars are designed with internal vibration absorber mechanisms that are not tunable. These anti-vibration bars will be referred to as AVB bars.
However, even tunable boring bars and AVB bars do not produce satisfactory performance for boring bars with narrow diameter and longer length. This limitation is problematic since, for certain cutting applications, narrow long length boring bars are particularly desirable. Steel tunable boring bars are generally only effective for Length/outer Diameter (L/D) ratios of less than about 10. Also, steel tunable boring bars even when dynamically stable may not have high enough static stiffness to prevent deflection, which may limit the ability of achieving desirable dimensional part quality. When necessary, for some applications, tunable boring bars with a solid carbide shank may be used if higher L/D ratios are required. However, as described above, solid carbide is expensive, heavy, and brittle, making it less useful for certain applications. Therefore, there is a need for a tunable boring bar that reduces vibration to provide improved performance for boring bars having high L/D ratios, and preferably for L/D ratios of 10 and greater. The boring bar should be capable of use with presently available toolholder assemblies and dynamic tuning arrangements. The tunable boring bar of the present invention provides some or all of these features.